highlight utility skills (cleanse, interrupt, ...) when effective

[Jeeqbit]

As you said, stuns would highlight all the time against trash and knockback immunity would be up either a lot or all the time because it's useful in other situations such as preventing cast interruptions or applying slow. Stuns can make you a better tank by reducing damage in old content from ARR or HW (where tanks have less self-heals) or to stop large circle aoes cast by trash, but aren't necessary.

Knockback immunity isn't necessary, even in savage, where you can go to the very edge or the middle to resolve knockbacks the way they are intended (and usually indicated in the normal version). This is useful to remember, actually, in case you ever use it early by accident in savage. The only time that might be a problem in savage is if the mechanic immediately following it requires everyone to be in a certain position and everyone else decided not to get knocked back with you.

Except for Doom, debuff cleansing isn't necessary either but people will appreciate it, especially if it impacts their rotation. Most of the annoying versions of these debuffs happen in ARR or when you fail a mechanic. Esuna needs a target, so even if it's highlighted, will they understand who to use it on?

Interrupting casts is usually not necessary but appreciated. Too many casts in ARR are interruptable and the ones in Shadowbringers are mostly on dungeon trash that an average tank won't notice because they are surrounded by 20 other enemies.

They are all things that make you a better player if you use them but that a lot of players just won't use right even if you handhold them.

Stunnable/interruptable stuff share the same indicator if you look at the cast bar, if I'm correct.

Not correct. An enemy that allows interrupts does not necessarily allow stuns, but sometimes they do and this can be used to interrupt multiple casts until interrupt is available again (unless a White Mage is spamming Holy ).